[MOC] Sun, earth and moon orrery

[EWay]

Very much jumping on the orrery bandwagon right now... I made this almost four years ago as a commission for my dad, but I wasn't active on EB back then (arguably I'm still not :D) so now seems like as good a time as any to share this!

This is far from the first Lego orrery as I'm sure everyone reading this is aware of by now, but I believe mine is unique in that the centre of rotation of the Earth-Moon system is offset by one stud. This because the Moon doesn't rotate around the Earth, in fact they rotate around each other (wikipedia: barycentre), but the centre of rotation just happens to be close to the centre of the Earth because the Earth has much greater mass.

You can see the barycentre offset in the next photo - there are two 8-tooth gears side by side under the black 1x7 liftarm; the one that's not directly beneath the Earth is the one around which the entire Earth-Moon system rotates:

The gear ratios are as follows (counting one model Earth rotation as one 'day'):
Moon completes one rotation around the Earth: 28 days (a synodic month*)
Earth completes one rotation around the Sun: 360 days (a year)
The sun rotates fully on its axis: 24 days **

* A synodic month should be about 29 days and a sidereel month should be about 27 days. A synodic month is one full lunar orbit with respect to the position of the sun, as viewed from Earth, whereas a sidereel month is one full lunar orbit with respect to the stars as viewed from Earth. There's a difference because the Earth is going round the sun and by the time the moon comes back around the Earth is not where it started. Comparing the moon to where the sun is is the equivalent of one full rotation of the small 28-tooth turntable that holds the Earth and Moon.

** Since the sun is a fluid, it doesn't rotate uniformly. The poles rotate roughly every 35 days and the equator rotates every 25 days. When I built this I used an incorrect source of information that told me it was 30 days at the poles and 24 days at the equator, so I chose 24.

Here's a video of it in operation: 

And there are loads more photos in the album: https://flickr.com/photos/eastawat/albums/72157713506039878/

 

Edited December 8, 2023 by EWay
video link

[aeh5040]

Can't have too many orreries! That's very nice and neat. I like the exposed gearing.

Of course strictly speaking you need to offset the sun slightly to account for the orbit of Jupiter 😉

Edited December 8, 2023 by aeh5040

[Davidz90]

Indeed, the more orreries, the better :) This one is nice and compact. Earth-Moon rotation offset is an interesting feature.

[R0Sch]

Wow, that earth is spinning fast! Good work with the accuracy of the rotation periods. I didn't even know Earth-Moon had a barycenter  that is offset by such a great distance until a couple of days ago.

[EWay]

3 hours ago, aeh5040 said:

Can't have too many orreries! That's very nice and neat. I like the exposed gearing.

Of course strictly speaking you need to offset the sun slightly to account for the orbit of Jupiter 😉

Good point, will make sure to do that if I ever build a full solar system one :D

[howitzer]

Very nice! I once wanted to build my own full solar system orrery, but couldn't figure out how to make all of the planets orbit around the same axis so I scrapped the idea... I guess that would become pretty large and complex, perhaps too much so to be doable with Lego.

[Davidz90]

12 minutes ago, howitzer said:

Very nice! I once wanted to build my own full solar system orrery, but couldn't figure out how to make all of the planets orbit around the same axis so I scrapped the idea...

Here is one from Lego Ideas (as I recall, it got rejected in the past, this is its 2nd or 3rd attempt, I think?). Full solar system, ingenious but quite bulky.

https://ideas.lego.com/projects/c600ea70-0e2d-4192-8bbc-79eecbc796e4

Edited December 8, 2023 by Davidz90

[JaBaCaDaBra]

1 hour ago, Davidz90 said:

Here is one from Lego Ideas

Oh wow, this one is really awesome.

[howitzer]

2 hours ago, Davidz90 said:

Here is one from Lego Ideas (as I recall, it got rejected in the past, this is its 2nd or 3rd attempt, I think?). Full solar system, ingenious but quite bulky.

https://ideas.lego.com/projects/c600ea70-0e2d-4192-8bbc-79eecbc796e4

Yeah, I remember that, It's a great one though indeed really bulky.

[Lego Tom]

On 12/8/2023 at 7:51 AM, R0Sch said:

Wow, that earth is spinning fast!

I noticed tiny people, cars and animals flying off! OK, maybe that was my imagination...

[Davidz90]

17 minutes ago, Lego Tom said:

I noticed tiny people, cars and animals flying off! OK, maybe that was my imagination...

Assuming that the earth is doing 4 rotations per second (looks like it in the video), on the equator they reach 50% of the speed of light 

[Lego Tom]

10 hours ago, Davidz90 said:

Assuming that the earth is doing 4 rotations per second (looks like it in the video), on the equator they reach 50% of the speed of light 

So if we were to speed it up to 9 rps or higher, we could prove Einstein wrong!